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outline Motivation Experimental Setup A nalysis

Indications for a structure near the 2 p threshold in the g p p gg reaction. David Trnka II. Physikalisches Institut, University Gießen for TAPS- and CB /ELSA -Collaboration. outline Motivation Experimental Setup A nalysis. Preliminary result from the WASA collaboration.

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outline Motivation Experimental Setup A nalysis

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  1. Indications for a structure near the 2p threshold in the gp pgg reaction David Trnka II. Physikalisches Institut, University Gießen for TAPS- and CB/ELSA-Collaboration • outline • Motivation • Experimental Setup • Analysis

  2. Preliminary result from the WASA collaboration • • recently peaklike structure in • pp  ppgg near 2p threshold • reported i.e. @ DPG/Koeln • • angular distrubution consistant • with s-wave production • • BG: • pp  pppp (15%) • • double bremsstrahlung (rest) • • artifact peak due to • merged clusters (1 count) • (2 g‘s from 1 p decaying • in a similar way) • • pp  ppp+p- (0%) • Possible Interpretation: •  peak structure due to 2 • photon radiation in the • mesonic s channel: • pp  pps  ppgg Also: CB @ LEAR reanalysing data

  3. Experimental Setup accelerator facility ELSA@Bonn side view of experimental area tagging system: taggingrange: 22 to 93 % of incident electron beam energy

  4. 4 p detector system CB/TAPS @ ELSA Crystal BarrelTAPS 1290 CsI 528 BaF2 side view front view of TAPS g f = 360° q = 30°to 168° f = 360° q = 5°to 30°

  5. Analysis • mesons identified via invariant mass analysis of decay photons • all beam energies allowed (ca. 500 MeV to 2.5 GeV) • only tiny fraction of data analysed • clean up spectra via exclusiv measurent require coplanarity apply missing mass cut (p treated as missing particle)

  6. a quick look comare to WASA Tp = 1.2 GeV and 1.36 GeV in pp reactions corresponds to Eg = 0.67 GeV and 0.77 GeV in g p reactions low beam energies: • peak structure around 2mp visible • for solid target structure even below 2mp

  7. a bit more sophisticated look • scan through different beam energy bins • remove/reduce merged cluster candidates  cut on clustersize ( < 10 crystals)  cut on opening angle of the photon pair (>20°) • investigate pp background bg not yet understood (FWHM = ?)  slight shift of peak position with beam energy  more detailed study would be great (CB/TAPS @ MAMI)

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